Proton-Controlled Electron Injection in MoS During Hydrogen Evolution Revealed by Time-Resolved Spectroelectrochemistry.

Monolayer MoS is an effective electrocatalyst for the hydrogen evolution reaction (HER). Despite significant efforts to optimize the active sites, its catalytic performance still falls short of theoretical predictions. One key factor that has often been overlooked is the electron injection from the conductive substrate into the MoS. The charge transfer behavior at the substrate-MoS interface is nonclassical, exhibiting a liquid-gated electron injection behavior, the underlying mechanism of which remain under debate. To investigate this, we employ nanosecond time-resolved spectroelectrochemistry to probe the electron injection dynamics into monolayer MoS under operando HER conditions. Simultaneously, transient current measurements provide insights into the electron density at the substrate. By combining the electron density obtained from the MoS through spectroelectrochemical analysis with the electron density at the conductive substrate derived from transient current measurements, we explore the electron injection dynamics and characterize the current density potential (J-E) behavior at the substrate-MoS interface. Our findings show that the electron injection barrier and capability correlate strongly with proton concentration in the electrolyte. This relationship likely reflects the electron concentration-dependent conductivity of MoS, where higher proton concentrations lead to fewer stray electrons before injection begins.